Previous Window brings up the last window that was active, regardless of application. Previous Window brings up no UI, and can be invoked with a single keyboard shortcut.
Basic Use:
To switch to the previously used window press the Ctrl-Shift-` keyboard shortcut, or select the Previous Window menu item from the Actions menu in the global application menu.
The keyboard shortcut for Previous Window can be configured in the Shortcuts pane of the Preferences window.